1. Field of the Invention
This invention relates to silicon wafers for producing oxide layers of high breakdown strength and a process for the production thereof.
2. Description of the Prior Art
With increasing miniaturization in the production of electronic components, increasing requirements are imposed on the silicon wafers used as a starting material. In this connection, the production of thin oxide layers having high breakdown strength is increasingly important. The main reason for this is that, in particular with highly integrated components, thin oxide layers are more frequently provided as structural elements which layers are required to have an excellent breakdown strength.
Such oxide layers are frequently described in the art as "gate-oxide". In most cases, these oxide layers are produced in the surface region of silicon wafers which have, in their interior, regions formed by a preceding treatment which possess gettering action. These interior regions are adjacent at least one surface zone free of such gettering centers which is in general up to about 50 .mu.m thick and in which the structures actually active in the production of the components are laid down.
An example of a treatment for producing such characteristics is the precipitation of the oxygen contained in the wafers, which is used in particular in silicon obtained by the Czochralski crucible pulling technique. In this material the oxygen content is typically in the range of 10.sup.14 -2.times.10.sup.18 atoms/cm.sup.3. In this case, a temperature treatment produces oxygen precipitations (so-called "precipitates") in the interior of the wafers. These oxygen precipitations, because of their intrinsic gettering action, are of great importance for the processes which are used for the actual component production, which processes take place in the peripheral zone of the wafers which is free of such precipitates ("denuded zone"). These relationships and procedures are known and are explained in more detail, for example, in the paper entitled "Advanced CZ crystal production for VLSI applications" by H. Walitzki and M. Blatte, published in VLSI Science and Technology (1984), pages 10-18, and also in the other literature cited in that article.
Apart from the formation of oxygen precipitates, other known processes for producing intrinsic getters in silicon wafers are insignificant and are only occasionally used. These processes are, for example, the controlled implantation of foreign ions such as hydrogen or nitrogen, which are occasionally used for example in the case of low-oxygen material (typical oxygen content 10.sup.12 -10.sup.16 atoms/cm.sup.3), such as is obtained, by crucible-free zone pulling.
Regardless of the method by which the regions with intrinsic gettering action are produced in a particular case, the average breakdown strengths can, however, be increased reproducibly and with satisfactory yield only to certain limit values where silicon wafers are produced and pretreated in the usual manner. The typical values at which the oxide layers lose their insulation capability for the standard silicon qualities vary, depending on the respective test methods used. Such methods are explained, for example, in Philips J. Res. 40, No. 3, 1985.